GEOCHEMISTRY. - From γῆ, "earth," and γυμνῆ, "chemistry." It is the science that studies the physical-chemical composition of the earth. From the time when, in an infinitely remote past, a fragment of solar matter broke away from the intensely burning mass of the star of our system to form the earth, an infinite series of chemical and chemical-physical reactions have agitated the chemical elements present in solar matter; various compounds have been born and have disappeared; immense masses have passed from gaseous to liquid, then to solid, and then, in part, have been re-melted; the gases of the primordial atmosphere have reacted with solids and with liquids; the first solutions have washed away the solid masses or have attacked them; plant and animal organisms, having appeared very recently in the play of reactions, then contributed to the formation of the present equilibrium. And the reactions continue: even now entire mountain chains are slowly being destroyed by the action of geological agents, and new sediments are forming within the waters while lava masses cool and the degassing of magmas continues.
Well then, geochemistry investigates all these chemical and chemical-physical reactions that constitute the present and past life of our earth, reactions that describe the genesis of our planet and its present chemical-physical state and allow, in part, the prediction of the future development of the chemical processes that will take place upon it.
Geochemistry is rightly considered a science of the 20th century: it was born from the stock of geological doctrines to which it belongs and indissolubly binds mineralogy and geology to chemistry, re-establishing those relationships that in the most markedly naturalistic period of the geo-mineralogical sciences seemed to have weakened.
The theories that until recent years had guided the investigations of geochemists admitted that during cooling the primitive matter had differentiated by gravity, letting fall toward the center an alloy of iron and nickel, nife (from Ni = nickel and Fe = iron), supposed to constitute the earth's core or siderosphere, and depositing above it first a layer of sulfides or chalcosphere and then the surface layer or lithosphere, consisting predominantly of silicates. The lithosphere, only 30-80 km thick, was divided into two portions: the sima (from Si = silicon and Mg = magnesium), formed of basic rocks, that is, rich in silicates of iron and magnesium, and the sial (from Si and Al = aluminum), lighter, with aluminum silicates and free silica, that is, essentially formed of rocks of the granite type.
As a consequence of this division of the earth into layers, the elements were also divided into atmophile, lithophile, chalcophile and siderophile, according to whether they abounded in the atmosphere or in one of the earth's layers.
The hypothesis on the constitution of the deep layers had been reached by considering the difference between the average density of the earth and that of surface rocks, the constitution of meteorites, and the propagation of seismic waves.
Now geochemists are orienting themselves toward a new theory, more in conformity with present knowledge; according to it, the matter that had broken away from the sun began to differentiate through cooling, immediately lost great quantities of hydrogen, helium and other light elements, and thus came to be enriched at the surface with heavier elements, while new conveyances of gaseous masses arrived at the surface, agitating the differentiated matter. Further cooling led to the condensation of the less volatile masses and to the formation of a liquid surface shell consisting of ferro-magnesian silicates and having, on the whole, the chemical characteristics of an alkali-basaltic magma. It should be emphasized that, according to this theory, the degassing and growing heavier of the surface masses caused them to sink, but only to the depth compatible with the viscosity of the matter which, approaching the center, was ever more complex and condensed and therefore more viscous. Consequently, beneath the liquid shell there remained a core of virgin, non-degassed and supercompressed solar matter.
No general differentiation by gravity or by fractional crystallization has therefore occurred, and hence the present granitic masses would have formed not by differentiation of the primitive magma but by re-melting of sediments.
In fact, above the liquid shell there was the pre-geological atmosphere consisting of water vapor, nitrogen, hydrofluoric acid, hydrochloric acid, carbonic acid, and of alkaline chlorides and fluorides, all perhaps at 400 atmospheres of pressure. The further lowering of temperature led to the consolidation of the magmatic mass, while through pneumatolytic reactions deposits with acid silicates and quartz were formed; below the critical point of water, solutions rich in acids began to act which, attacking the surface shell, determined the formation of argillaceous and siliceous sediments; and it is these masses rich in silica, formed in the pneumatolytic phase and then in the hydrothermal phase, that—accumulated by waters in depressions and then brought by sinking into zones at higher temperatures—have been re-melted and have given origin to many of the present granites.
From investigations of the solar atmosphere it results that in it, for every hundred atoms of silicon, there are: hydrogen 150,000-300,000 atoms, helium 9,000, oxygen 5,000-10,000, carbon 1,500-3,000, nitrogen 500-1,000, fluorine 500-1,000, magnesium 100-1,000, iron 84, sodium 82, potassium 24, manganese 7, nickel 4.8, cobalt 2.9, chromium 2.2, boron 1, aluminum 0.5-3, calcium 0.4-1.8, vanadium 0.5, zinc 0.3, copper 0.2, titanium 0.03-0.25; all the other elements are present in very small quantities. The principal constituents of solar matter, excluding those that are gaseous under our environmental conditions, are all found on the earth, while in general the minor constituent elements of the sun are also present on the lithosphere.
The first quantitative investigations based on numerous analyses of surface rocks led to the observation that only eight chemical elements (oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium) participate in the formation of the surface crust with percentages by weight greater than 1% (among them oxygen constitutes 49.5% by weight of the crust and silicon 25.3%); four elements (hydrogen, titanium, chlorine and phosphorus) participate with percentages between 0.1 and 0.9%, and only thirteen others (manganese, carbon, sulfur, barium, chromium, nitrogen, fluorine, zirconium, strontium, nickel, vanadium, zinc, copper) with percentages between 0.1 and 0.01%. The enormous majority of the elements is present only in traces on the surface; in fact, the sum of the percentages relating to the twenty-five elements cited amounts to 99.96%. If one then takes into account the volume that the atoms (or ions) occupy in the crystals of the minerals of the rocks, one arrives at the unexpected result that oxygen alone occupies 91.77% of the volume of the lithosphere.
The detailed study of the distribution of the elements on our planet can be divided into three parts: the 1st distribution process occurred during the formation of the first high-pressure atmosphere and of the fused surface crust, with the trapping, in the center of the mass, of supercompressed virgin solar matter; it essentially led to the separation from the surface matter of helium, hydrogen, carbon and nitrogen and to the fixation of great quantities of oxygen in the silicates and oxides of the liquid mass and in water vapor.
In the 2nd process of geochemical distribution of the elements, which took place during the consolidation of the superficial molten mass, all the minerals (almost always silicates) of the fundamental constituent elements of the magma (aluminum, iron, calcium, sodium, potassium, and magnesium) were separated, and the minor constituents (the other chemical elements of the lithosphere) were divided approximately into two categories: those which, by their dimensions (ionic or atomic radius), could be hidden in the principal minerals of the rocks being formed (that is, they could enter their crystals as isomorphic substitutes for the fundamental constituents), which were diluted in the solid mass during its crystallization, and the others, the free elements, which could not enter the crystals of the common minerals and which accumulated in the residues of crystallization, giving rise to minerals of their own in a pegmatitic-pneumatolytic or hydrothermal phase.
Finally, the 3rd process of geochemical distribution of the elements takes place during the transformations that the minerals of the lithosphere have undergone and are undergoing during the processes of attack and transport by geological agents or during metamorphism.
These problems of the chemical constitution of the earth and of the distribution of the elements are the fundamentals of geochemistry, and here they can only be touched upon. Many other problems are linked to them, among which are those of bio-geochemistry, that is, investigations into the cycles traversed by the chemical elements when they become part of animals and plants and of their vital processes, those of the distribution and transformation of radioactive substances, and those of the conditions of genesis of individual minerals and then of rocks.
Bibliography
See also the series of works published by V. M. Goldschmidt and collaborators in Nachrichten der Gesellschaft der Wissenschaften zu Göttingen, Fachgruppe IV (Mineralogie und Geologie), 4 (1930), 6 (1930), 11 (1931), 12 (1931), 16 (1931), 18 (1932), 20 (1932), 25 (1932), 26 (1932), 27 (1932), 31 (1932), 33 (1933), 35 (1933), 36 (1933), 37 (1933), 40 (1933), 2 (1934), 4 (1934), 11 (1935), 15 (1936). F. W. Clarke, The data of Geochemistry, in United States Geological Survey Bulletin, 49 (1911).
A. Rittmann, Vulcani: attività e genesi, Naples 1944; id., Über die Herkunft der vulkanischen Energie und die Entstehung des Sials, in Geol. Rundschau, 30 (1938), p. 52; W. Kuhn and A. Rittmann, Über den Zustand des Erdinnern und seine Entstehung aus einem homogenen Urzustand, ibid., 30 (1941), p. 215; W. Kuhn, Stoffliche Homogenität des Erdinnern, in Naturwissenschaften, 30 (1942), p. 689; H. Haberlandt, Die Bedeutung der Spurenelemente in der geochemischen Forschung, in Monatshefte für Chemie, 77 (1946), p. 293; G. Carobbi, Proposte per un sistema cristallochimico degli ioni, in Atti e memorie dell'Accademia di Scienze, lettere ed arti di Modena, 5ª serie, 7 (1947); H. Haberlandt, Über die gesetzmässige Differentiation von Spurenelementen in Mineralien, in Tschermaks mineralogische und petrographische Mitteilungen, 1 (1948), p. 124.
Guido Carobbi